Various methods are known for fusing toner particle images on substrates. In conventional fusing systems, one or both of the fuser roller and the pressure roller may be heated and are somewhat compliant to create a wide nip to allow sufficient heating area. Such conventional fusing systems typically provide gloss levels less than about 20 at a 20° measurement measured by the Glossgard II 20° glossmeter as discussed below. Also when using coated papers, the wide nip causes overheating and thereby contributes to blisters as the receiving sheet leaves the nip. Unfortunately, the wide nip prevents obtaining sufficiently high pressure to remove the toner image relief in these materials.
Finishing color images of fusible toner particles has been attempted in typical fusing systems. In these fusing systems, as noted above, typically the gloss is relatively low. As a result systems for fusing colored images using methods and apparatus that are appropriate for fusing the black images to the substrate, do not provide the desired gloss. Alternate methods have been used to produce enhanced gloss images by fusing the toner particle images and, thereafter transporting the substrate bearing the fused toner image through a cooling zone and then passing the cooled substrate bearing the fused toner image to a release zone where the cooled substrate bearing an enhanced gloss image is released from the transport. In typical fusing processes it has been found that when conventional toners are used, the use of release additives such as silicone oil are required. The oil results in the presence of defects in the color image and in the surrounding area of the substrate when the alternate methods are used. There are a variety of reasons for these defects and it is considered that certain of these defects relates to the formation of a haze, which is a low color saturation area or dot in the image visible from certain viewing angles and under certain lighting conditions. This defect results in lower gloss and reduced image density.
A second defect resulting from the presence of the release oil is oil-laden images (ghosts). The oil presence on an imaged and fused sheet diffuses unevenly into the sheet fibers. Therefore when such a fused sheet comes in contact with a glossing belt, it leaves an oil imprint relating to the image on the belt, which is picked up by the following sheet showing a ghost image of the images of the preceding sheet. Ripples and wiggles are also attributed to the presence of release oil on the sheet since it reduces friction on the belt glosser nip and therefore can cause image ripples or wrinkles in the sheet.
Various processes for using belt fusers to produce higher gloss images on substrates are shown in U.S. Pat. Nos. 5,089,363; 5,256,507; 5,258,256; and 5,778,295. These patents are hereby incorporated in their entirety by reference and disclose apparatus and methods for the use of belt fusers to improve the gloss of the image on substrates especially in, but not limited to, colored images. These references also disclose various materials conventionally used in such processes.
Accordingly, a continuing effort has been directed to the development of a belt glosser to fuse, cool and release a substrate bearing a pre-fused toner image so that the gloss of the image may be enhanced without the need for or the detrimental effects of release oil.
According to the present invention, an enhanced gloss image is obtained without the use of, or detriments induced by, release oil by selectively producing an enhanced gloss electrophotographic toner image on a substrate by passing a substrate bearing a pre-fused image of colored fusible toner particles through a downstream glossing device. When the glossing device is used, color fusible toner particles are covered by a clear toner layer, that does not require use of a release oil to prevent offset. The clear toner includes from about 2.5 to about 10 weight % of an aliphatic, olefinic, hindered or unhindered ester wax having a molecular weight of typically less than about 2000. This material serves readily to prevent the clear toner from adhering to the surface of the fuser roller of the color printer and of the glossing device, and produces a high gloss image without defects.